This invention relates to electronic instruments for detecting and/or measuring electrical signals and, more particularly, to electronic instruments for detecting and/or measuring the frequency spectrum of electrical signals. Specifically, one embodiment of the invention provides an unbounded marker particularly adaptable for use in an electronic instrument known as a spectrum analyzer for measuring frequency and amplitude of electrical signals. The unbounded marker in accordance with one embodiment of the invention can facilitate a measurement by automatically re-tuning the spectrum analyzer to a new frequency range, thereby improving operator efficiency and enhancing overall measurement throughput.
Signal analysis, simply defined, is the extraction of information from an electrical signal, whether performed in the frequency or time domain. The most common time domain signal analyzer is the oscilloscope. In the frequency domain, the signal analyzer is the network analyzer or spectrum analyzer. These analyzers typically display the raw, unprocessed signal information, that is, voltage, power, period, waveshape, sidebands, and frequency.
By way of example, the spectrum analyzer is widely accepted as a general purpose test instrument capable of performing a broad range of measurements in the frequency domain. Generally, a spectrum analyzer is a scanning receiver that displays power and modulation characteristics of electrical signals over a selected frequency band. Examples of such spectrum analyzers are the HP 8568 and HP 8566 spectrum analyzers, the HP 8590 series spectrum analyzers, and the HP 71000A Modular Spectrum Analyzer available from Hewlett-Packard Company, Palo Alto, Calif.
One technique to perform frequency domain measurements with a spectrum analyzer is known as the swept-tuned technique. The swept-tuned frequency spectrum analyzer can be either a tuned filter or a heterodyned receiver.
Swept-tuned spectrum analyzers are used to measure a variety of characteristics of signals. There are many measurements which can be performed with a spectrum analyzer in response to a transmitted or received signal, where measurement of frequency, power, distortion, gain, and noise characterize a transmitter or receiver system.
FIG. 1 shows a generalized superheterodyne swept-tuned spectrum analyzer. An incoming signal mixes with a local oscillator (LO) signal, and when a mixing product equals the intermediate frequency (IF), this signal passes through to a peak detector. The peak detector output is amplified to cause a vertical deflection on a CRT display. The synchronism between the horizontal frequency axis of the CRT display and the tuning of the local oscillator is provided by a sweep generator which both drives the horizontal CRT deflection and tunes the LO.
Considered in more detail, the swept-tuned spectrum analyzer provides real-time, frequency-domain scans (sweeps) over a wide band of the frequency spectrum. The results, called a trace, such as the trace 2 shown in FIG. 1, are displayed on a labeled graticule 4 so that the frequency of individual signals and their corresponding amplitudes can be determined.
Test instruments with graphic displays, such as oscilloscopes, network analyzers, and spectrum analyzers, typically have user controls on a front panel to adjust the parameters of the measurement being performed. The graphic display provides the result of the last measurement that was performed. When the operator adjusts a control setting by means of a knob or button, a new measurement is performed, and the graphic display is updated to reflect the new measured data.
A marker 6 (or multiple markers) can be positioned on the trace for more accurate reading of the frequency and/or amplitude at a given point on the trace. Generally, the marker is the primary method of making measurements using a spectrum analyzer.
On known spectrum analyzers, the marker is adjusted up and down in frequency and is bounded by the start and stop frequencies of the given scan. In order to measure outside the given scan-width, the operator must re-tune the spectrum analyzer and then re-activate the marker before continuing with measurements. This sequence of operator actions, namely, move the marker, re-tune the spectrum analyzer, re-activate the marker, and move the marker, leads to inefficient operation. This is particularly true during the measurement of an unknown spectrum, such as during signal monitoring or electromagnetic emissions measurements.
One solution to this problem is having separate points of control for tuning and the marker. For example, two separate knobs can be provided on the spectrum analyzer. The operator uses one to adjust the tuned frequency and the other to adjust the position of the marker. This solution is better than the traditional marker but still requires the operator to move his hand from one knob to the other in order to perform the measurement.
Additionally, European Patent Application 88103406.0 discloses a "partially scrolling function." This patent application describes a tuning algorithm for speeding up data acquisition and display during tuning of a spectrum analyzer and improving the accuracy of the data acquired. In known spectrum analyzers, data is acquired a single sweep at a time, starting at a fixed start frequency and ending at a fixed stop frequency. The problem that occurs is that when the center frequency of the sweep is changed, there is no data available to take the place of the spectrum being scrolled into view, and, secondly, the sweep typically re-starts at the start of the frequency span and proceeds from lower to higher frequencies. This means that there is a time lag in gathering the new data, because the new spectrum can be swept last, and, also, there is either a flat line or invalid data in the new spectrum. The "partially scrolling function" sweeps the new spectrum first, either sweeping low to high for left scrolling or, for right scrolling, high to low. In this manner there is never invalid data presented or a time lag while a new sweep is performed. However, the "partially scrolling function" is a tuning function. It is not related to use of a marker. It is a method for more quickly and accurately tuning a spectrum analyzer. It does have value in speeding up measurements and assuring that the on-screen trace data is accurate. However, the frequencies and amplitudes of the data scrolled on the screen is raw trace data. It has the problem which all trace data does where the operator cannot easily find the frequency and amplitude of a given signal. Trace data requires visual interpolation between graticule lines along both the frequency and amplitude axes. Hence, a marker is additionally required in connection with the "partially scrolling function." Thus, the "partially scrolling function," for the sake of comparison to a marker, is simply a variation on tuning the spectrum analyzer and has the same drawbacks that tuning in a markerless system does. It would therefore be desirable to provide an unbounded marker that does not require the operator to perform unnecessary actions required during use of a traditional marker or dual front panel controls and which can automatically re-tune the spectrum analyzer to a new frequency range as in the case of the "partially scrolling function." Such an unbounded marker would facilitate the measurement process.